Plasmodium vivax (Pv) is the second most common cause of malaria in humans, being responsible for an estimated 70-100 million cases annually. Outside the African continent Pv is the most widely distributed malaria parasite. It co-exists with P. falciparum (Pf) and is responsible for significant morbidity and a substantial economic burden. Pv is responsible annually for as many cases of malaria in travelers as is Pf, the most common cause of malaria in the world. During the last decade, drug resistant Pv emerged on the island of New Guinea and has spread to other areas. There is a huge potential market for a Pv vaccine in travelers and military from the developing world, and among populations in countries with endemic Pv. Most malaria vaccine development efforts have been focused on developing a Pf specific vaccine. In contrast, Pv vaccine research has been neglected, mainly because of technical restrictions imposed by the lack of capacity to culture Pv in vitro. However, the fact that Pf and Pv co-exist in most malaria endemic areas worldwide presents technical and ethical constraints for the deployment of a vaccine effective only against Pf. During the past several yrs our partners in Colombia have concentrated significant effort on Pv studies. They have established the capacity to produce Pv spz, to successfully challenge Aotus monkeys with these spz, and to conduct Pv vaccine trials in Aotus monkeys and humans. They have also identified new domains containing functionally important antibody and T cell epitopes on a major candidate Pv antigen, the circumsporozoite protein (PvCSP). Here we propose to produce, purify, and biochemically characterize a set of rec proteins based on the PvCSP, and assess them for immunogenicity and protective efficacy in Aotus monkeys. The PvCSP is chosen because its Pf orthologue (PfCSP) is the only Pf protein ever shown to reproducibly protect humans, a monoclonal antibody against PvCSP protects monkeys in passive transfer, and for commercial reasons we believe the vaccine must be at a minimum directed against the pre-erythrocytic stages of Pv. Because of its capacity to produce large quantitities of secreted, easily purified rec proteins, the Pichia pastoris expression system will be used to produce the Pv CS proteins. These will encompass the entire amino and the entire carboxy regions of the PvCSP plus the two known major variant forms of the PvCSP repeat domains, thereby including all known antibody and T cell epitopes on the two major variants of the PvCSP. No such PvCSP immunogens have ever been produced or tested. This project will provide rec proteins and process development information that can be used for the choice of manufacturing under GMP conditions a rec protein for clinical trials. The association between Protein Potential (rec protein production, process development, and transition to GMP manufacturing, malaria vaccine development and clinical trials) and the group in Colombia (immunological expertise regarding PvCSP and capacity to conduct Aotus monkey protection studies, and human clinical trials), provides the capacity to take these proteins rapidly from concept to the clinic and field and eventually to the large market for a Pv vaccine.